1. Field of the Invention
This invention relates to the improvement of plants by the modification of lignin biosynthesis, particularly, but not exclusively, the improvement of digestibility of fodder crops.
2. Background Information
Grassland farmers, and farmers of other fodder crops, face a difficult decision each year about when to cut their crops for conservation. All grass varieties of agricultural importance suffer from the disadvantage that during the normal increase in dry matter yield with growth, the digestibility decreases. The farmer, therefore, has, to compromise between a lower yield of highly digestible material and a higher yield of less digestible material. Another limitation is that harvesting at optimum maturity may be prevented by unfavourable weather. If the decline in digestibility could be controlled or delayed, higher yields of highly digestible material could be obtained and the prevailing weather conditions would not play such a major role in determining the quality of the harvested crop.
Digestibility of fodder crops is determined, among other factors, by the amount and quality of lignin deposition which has taken place during growth of the plants and the degree of secondary modification of lignin deposited. Beside cellulose and other poly-saccharides, lignins are an essential component of cell wall in tissues like the sclerenchyma and the xylem of vascular plants. They play an important role in the conducting function of the xylem by reducing the permeability of the cell wall to water. They are also responsible for the rigidity of the cell wall, and, in woody tissues, they act as a bonding agent between cells, imparting to the plant a resistance towards impact, compression and bending. Finally, they are involved in mechanisms of resistance to pathogens by impeding the penetration or the propagation of the pathogenic agent.
Lignins are not only important in the productivity and performance of field crops but are also of great importance in trees for paper making. Considerable energy and chemical input is required to loosen, dissolve and remove lignin from the cellulose fibre which is required for paper making. In addition to these instances in which lignins present a constraint on the use of crop plants, lignins are also used as feedstocks for the preparation of speciality chemicals such as phenolics which can be used as precursors in chemical synthesis. Thus lignins and their biological and chemical modification are important.